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Star crust is 10 billion times stronger than steel

By Rachel Courtland

The crust of neutron stars is 10 billion times stronger than steel, according to new simulations. That makes the surface of these ultra-dense stars tough enough to support long-lived bulges that could produce gravitational waves detectable by experiments on Earth.

Neutron stars are the cores left behind when relatively massive stars explode in supernovae. They are incredibly dense, packing about as much mass as the sun into a sphere just 20 kilometres or so across, and some rotate hundreds of times per second.

Because of their extreme gravity and rotational speed, neutron stars could potentially make large ripples in the fabric of space – but only if their surfaces contain bumps or other imperfections that would make them asymmetrical.

A number of mechanisms have been proposed to create these bumps. The stars could, for example, gobble up material from a companion star. Bulges could also bubble up over hotter areas of the stars.

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In theory, these bulges could be stable on the outer surface of the star. Neutron stars are thought to be made up of a soup of neutrons covered with a solid crust. The crust is composed of crystals of neutron-rich atoms.

“But one of the big unknowns for all that work is the strength of the crust. Can you really support a mountain, or will the crust just collapse under the weight?” says Charles Horowitz of Indiana University in Bloomington.

Few defects

Since laboratory experiments cannot replicate the extreme conditions on the surface of a neutron star, astronomers have largely assumed that the crust’s strength would be similar to that of the strongest substances on Earth.

But in new computer simulations, Horowitz and Kai Kadau of the Los Alamos National Laboratory show the crust of a neutron star is much stronger.

Materials like rock and steel break because their crystals have gaps and other defects that link up to create cracks. But the enormous pressures in neutron stars squeeze out many of the imperfections.

That produces extraordinarily clean crystals that are harder to break. A cube of neutron star crust can be deformed by 20 times more than a cube of stainless steel before breaking.

Breaking point

But the atoms in neutron star crusts are pulled together much more tightly than in steel, so it takes 10 billion times as much pressure to push it to the breaking point, Horowitz told New Scientist.

Benjamin Owen of Pennsylvania State University in University Park says the simulations firm up previous suspicions that neutron star crusts might be stronger than astronomers had been estimating. “There was sort of some hand-wavy hints about that a few years ago, but this is really the first thorough calculation,” he says.

The stronger crust means a neutron star can support a larger bulge than thought – a “mountain” could rise some 10 centimetres above the surface, stretching over several kilometres.

Stronger signal

Now, “all else being equal, the maximum height of a ‘mountain’ on a neutron star is now 10 times what we thought,” Owen told New Scientist.

That would produce gravitational waves with 100 times the energy as those previously calculated, which could boost the likelihood that ground-based experiments like the US Laser Interferometer Gravitational-Wave Observatory (LIGO) could spot the signals, he added.

The simulations could also shed light on starquakes, the reverberations triggered when intense magnetic fields tear open the crust of a neutron star. A stronger crust means these quakes can produce even more energetic gamma-ray flares and gravitational waves, Owen says.